This invention relates, in general, to reciprocating drive mechanisms for dot matrix printers, and more particularly, to a reciprocating drive mechanism for high speed dot matrix printers having a plurality of print wires or hammers assembled in a print bar assembly for horizontal line dot matrix printing.
There are a variety of methods and apparatus for imprinting alphanumeric characters or other information onto paper or other printing medium. Some of which would include daisy wheels, ink jets, electrophotographic and dot matrix printers. Dot matrix printers are generally available in two different design classifications, the first being serial printers, and the second, line printers. Serial printers usually have a cluster of selectively operable print wires which sequentially form independent and complete alphanumeric characters as the print wire cluster assembly is moved across the page from left to right and back. Dot matrix line printers utilize a temporary printer memory into which is transferred a complete line of information to be printed. The print bar assembly normally contains a plurality of selectively operable print wires or hammers arranged in a single horizontal row and is utilized to print a complete horizontal line of dots which, when all of the horizontal lines of the intended matrixes for that line of information are printed, will form a complete line of information.
Horizontal line, dot matrix, printers are the printers of choice for high speed printing applications. The print bar assemblies normally contain a large number, typically ranging from 33 to 132, of selectively operable print wires or hammers for printing each horizontal line of the desired dot matrix for each line of information to be imparted to the printing medium. The large number of print wires or hammers is necessary in order to minimize the horizontal travel of the print bar assembly, thus minimizing travel time across the page and increasing the speed at which the information is printed. Print bar assemblies for high speed dot matrix line printers, having a large number of print wires or hammers in the assembly, represent a substantial mass to be oscillated back and forth. Typical oscillation rates range from 15 Hz. to 80 Hz. At these oscillation rates the vibration induced in the printer by the oscillation of the print bar assembly is significant and represents a substantial design problem. As a result, high speed dot matrix line printer frames are usually rather stoutly constructed. A variety of mechanical designs have been developed to incorporate some sort of a reciprocating counter mass into the printer frame design so as to reduce the mechanical vibration caused by the oscillation of the print bar assembly.
Another problem with the current designs is that, unless the desired oscillation rate can be matched with a resonant frequency of a mechanically sprung print bar assembly, a significant amount of force is needed to drive the print bar assembly at the desired oscillation rate. This has resulted in substantial efforts to design print bar assembly suspension systems which will provide the desired resonant frequency, and, control systems for maintaining the oscillation rate of the print bar assembly at the desired frequency.
Pennebacker, U.S. Pat. No. 4,227,557, discloses a suspension arrangement for a high speed printer having two print bar assemblies, each mechanically sprung to an intermediate frame which, in turn, is mechanically sprung to the main printer frame. The electrical control systems of Pennebacker is designed to monitor and maintain each of the two print bar assemblies at the same resonant frequency in reciprocal oscillation to each other so as to minimize the force required to drive the printer and also to counter balance the vibrations induced by the oscillation of each of the individual print bar assemblies.
Matsumoto, et al., U.S. Pat. No. 4,421,430, discloses a printing mechanism which is provided with a counterbalanced hammer bank such that the hammer bank and a counterweight are oppositely reciprocated by a pair of coaxial identical, orthogonally oriented cams. This is a mechanical drive system which attempts to insure a reciprocal oscillation of the print bar assembly and counterweight, but is difficult and expensive to produce and is subject to wear in high speed dot matrix line printer applications.
Mayne, et al U.S. Pat. No. 4,463,300, discloses a linear motor digital servo control system in combination with a print bar assembly which is allowed to bump into shock absorbing mechanical springs attached to the print bar assembly at each end of the print bar assembly's line of travel.
One of the most common embodiments in use today utilizes a design wherein a print bar and a counterweight are each mechanically sprung, by the use of leaf springs, from the print bar assembly. A linear motor is mechanically connected between the two mass assemblies and is used to drive each, simultaneously, in reciprocal oscillation to the other. A typical embodiment of this design is shown in FIGS. 1 of both Khorsand, U.S. Pat. No. 4,599,007 and Miller, U.S. Pat. No. 4,637,307. This design is also shown, for purposes of comparison with our new design, in FIGS. 2 through 5 of this specification.
A major problem with the design disclosed by Khorsand and Miller is that substantial care must be taken to insure that the spring rate and the mass for the print bar assemblies are carefully matched to the spring rate and mass of the reciprocal counterweight to insure a single resonant frequency at the desired oscillation rate. The resonant frequency for a mass mechanically sprung to a fixed frame can be determined by the following mathematical formula: ##EQU1## Where F.sub.rn represents a resonant frequency, K equals the spring constant and M represents the mass being oscillated. Utilizing this formula, it becomes apparent that when two independently sprung masses are driven in reciprocal oscillation to each other it becomes necessary to match the masses and spring constants in order to develop a single resonant frequency. Mathematically represented, the matching formula is as follows: ##EQU2## Where M.sub.1 represents the mass of the print bar assembly, K.sub.1 the spring rate for the print bar assembly springs, and M.sub.2 the mass of the counterweight, which may include the the driver assembly, and K.sub.2 the spring constant for the counterweight springs. Matching these rates is a difficult, time consuming and often expensive process.
Another problem with the current design as disclosed in Khorsand and Miller is that this design results in substantial bending moments around the points of attachment of the springs to the printer frame. FIGS. 4 and 5 of this disclosure represent mechanical schematic representations of these bending moments for the designs disclosed in FIGS. 1 of Khorsand and Miller. FIGS. 4 and 5, as later described in this disclosure, are used for illustrative purposes and comparison with the design disclosed herein.
It is an object of this invention to provide an apparatus whereby the relevant mechanical spring connection between the reciprocally oscillating print bar assembly and counterweight is a mechanical spring assembly operatively connecting the two mass assemblies to each other as opposed to independently springing each to a fixed printer frame, thus eliminating the need to match masses and spring rates in order to obtain a single resonant frequency. A second object of this invention is to significantly reduce the bending moments imparted to the printer frame as a result of the reciprocal oscillation of independently sprung print bar assemblies and counterweights when in reciprocal oscillation.
A third object is to reduce the time and cost of manufacture of a high speed print bar assembly.